The New International Encyclopædia/Hour-glass
HOUR-GLASS. An instrument for measuring intervals of time. It is made of glass, and consists of two bulbs united by a narrow neck; one of the bulbs is nearly filled with dry sand, fine enough to run freely through the orifice in the neck, and the quantity of sand is just as much as can run through the orifice in an hour, if the instrument is to be an hour-glass; in a minute, if a minute-glass, etc. The obvious defects of this instrument are the expansion or contraction of the orifice produced by heat or cold, and the variations in the dryness of the sand, all of which produce deviations from the true measurement of time. Instruments constructed on this principle are still used by navigators in ‘heaving the log,’ for the purpose of measuring the time during which the log-line is allowed to run out. Similar instruments are also employed by cooks in fixing the proper time for boiling eggs.